Gypsum bowl for sunshine recorder, with lens consisting of a glass sphere filled with Canada balsam.

Possibly an experimental type of sunshine recorder used at Greenwich and Kew Observatories during the early 1870s.

Sunshine recorders are positioned on a local meridian (north-south line) and record the number of hours of sunshine. Each day, a new card strip is placed into the grooves behind the glass sphere. As the Sun crosses the sky, the sphere acts as a magnifying lens, burning a line into the card that’s specially designed to scorch rather than burst into flames. The strip varies in length and position according to the seasons.

The idea was first devised by John Francis Campbell in 1853 and was later developed by the physicist Sir George Gabriel Stokes in 1879. Although simple to use and easy to maintain, the instruments are susceptible to variable results and need human intervention to replace the card strip each day, hence modern meteorological observatories rely on electronic sensors instead.